In recent years, the information-oriented society has been quickly developed and the information recording capacity over 100 Gbytes has been required per 2.5-inch magnetic disk in magnetic recording apparatuses represented by HDDs (hard disk drives). In order to satisfy such a requirement in the magnetic disk, it is necessary to realize an information recording density over 200 Gbits per square inch (200 Gbits/inch2). For performing stable recording/reproduction at such a high recording density, it is generally preferable to employ a perpendicular magnetic recording system as a magnetic recording/reproducing system.
As a magnetic recording medium for use in the perpendicular magnetic recording system, for example, a granular medium is known which exhibits excellent recording characteristics (see, e.g. Non-Patent Document 1). Specifically, the granular medium is provided with a granular layer (Granular layer) having a structure in which magnetic grains are surrounded by a nonmagnetic substance. As the granular medium, there is proposed, for example, a structure in which an oxide such as SiO2 is added to CoCrPt (see, e.g. Non-Patent Document 2).
Further, conventionally, attempts have been made to improve the magnetic properties of granular layers from various aspects (see, e.g. Patent Documents 1 to 4). For example, Patent Document 1 discloses a structure in which the ratio of a nonmagnetic compound occupying the total volume of both a magnetic material and the nonmagnetic compound is about 2% or more and about 30% or less. Patent Document 2 discloses a structure in which a large number of magnetic grains are separated by a grain boundary layer and the average separation distance between the mutually adjacent magnetic grains along a straight line connecting the centers of gravity thereof is 1 nm or more.
Patent Document 3 discloses a structure in which a base material surrounding magnetic crystal grains contains a predetermined combination of elements. Patent Document 4 discloses a structure in which grain boundary regions of magnetic crystal grains contain an oxide of Ti, the substance amount ratio of the oxide of Ti in a magnetic recording layer is 5 mol % or more and 15 mol % or less, and at least TiO and/or Ti2O3 is contained as the oxide of Ti.
Further, conventionally, there has been proposed a CGC (Coupled Granular and Continuous) medium in which a continuous film layer (Continuous layer) is further provided on a granular layer (see, e.g. Patent Document 5). By providing the continuous film layer, it is possible, for example, to improve the thermal stability of recorded signals.    Non-Patent Document 1: Appl. Phys. Lett. 52(6). 8 Feb. 1988 pp 512-514    Non-Patent Document 2: T. Oikawa et al., IEEE Trans. Magn, vol. 38, 1976-1978 (2002)    Patent Document 1: JP-B-H8-7859    Patent Document 2: JP-A-2002-358615    Patent Document 3: JP-B-3725132    Patent Document 4: JP-A-2005-196813    Patent Document 5: Specification of U.S. Pat. No. 6,468,670B1